Thermoplastic polymers are used widely to manufacture articles such as pipes and pipe fittings which require good corrosion and chemical resistance, low weight and good fabricability for use in transportation of gases, liquids, solids, slurries or the likes under pressure and non-pressure conditions or for protection of sensitive components, such as fiber-optics or cables. These applications frequently require connections between pipes and/or other objects. The joining of articles made of thermoplastic material may be accomplished by mechanical means such as threaded connections, couplings, flanges; chemical means such as solvent cementing, or by thermal means such as fusion bonding
Mechanical joints generally work well for small diameter pipes and non-pressure applications, but they are not cost competitive or technically suitable for large diameter pipe and/or pressure piping systems.
Solvent cementing is widely utilized for some thermoplastics, for example for polyvinylchloride pipe and fittings. Typically solvent bonding utilizes a solvent-based primer or cleaner to prepare the surfaces to be bonded and a solvent cement system that contains solvent(s) and resin in conjunction with an interference fit joint. Solvent cements may be used without primers or cleaners, however, joint integrity may be compromised. However, interference fit joints limit the ability to accurately lay out the pipe and fittings prior to cementing. Adhesive primers are widely used with solvent-based cement systems for poly(vinylchloride) and chlorinated poly(vinylchloride) piping systems to insure acceptable joint bonding. However, primers release as much as 650 grams per liter of volatile organic compounds (VOCs) into the environment. Moreover, the bonding strength can be inconsistent and solvent-based systems do not work well with many thermoplastic pipes and fitting materials, for example polyolefins.
A common method of joining pipe is butt fusion welding. It is commonly used for large diameter pipes and relatively simple layouts. Three important factors effecting satisfactory butt fusion joints are absence of contamination, sufficient heat input, and good final morphology of the weld. Failure to achieve any of the above can lead to an unsatisfactory pipe joint, for instance pipe leakage. Further, butt fusion requires a large welding machine and generator.
A pipe may be thermally bonded to a fitting by providing a diametrically enlarged female portion on the fitting, heating the inside of the enlarged portion and the outside of the end of the pipe to fuse the thermoplastic on each, and forcing the pipe end into the enlarged portion while the thermoplastic is in a somewhat fluid state. Upon cooling, the fused thermoplastic materials join and bond the articles together.
However, when articles are joined by the conventional thermal bonding approach described above, the joint may not be satisfactory, as some of the fused thermoplastic may be forced into the inside of the fitting and pipe to impede fluid flow, and because there may be leakage paths through the joint resulting from the sticking of the fused thermoplastic to the tooling of the heating unit. Further, because specialized equipment is required, fusion bonding may be acceptable to the professional, but it is not practical for the increasing Do It Yourself (DIY) market.
There have been attempts to overcome the deficiencies of conventional thermal fusion bonding by providing an electrically resistant heating coil or element positioned adjacent to the inside or imbedded in the surface of the fitting to be welded. This process is known as electrofusion. However, if done improperly, incomplete fusion can result. Furthermore, thermal fusion bonding by any of the above methods is difficult or impossible for joining pipes or pipe systems made of different thermoplastic resins.
Conventional pipe joining methods may further require time consuming and costly surface treatments such as corona, flame, or plasma treatment to achieve strong and monolithic joints.
Accordingly, there has been a need for a process to form extensive, continuous, economical, and strong joints between thermoplastic, especially polyolefin, pipes and other pipes or objects with lower VOC emissions and which is convenient and economical. The present invention fulfills this need.